The non-contact measurement of three-dimensional (3D) objects to extract data regarding their physical form and location in space is a topic which has excited much research. Many techniques have been developed to suit the distance to the object, the precision with which the object's features need to be measured and so on.
One common technique is to illuminate the remote object with a light source of known pattern; so called “structured illumination”. A camera is located some distance away from the structured light source and arranged to collect an image of the projection of the structured light pattern onto the surface of the remote object. An image processor derives the 3D profile data by analysis of the deformation of the structured light pattern, which is representative of the distance to the remote object.
As this system works on trigonometric principles, the depth accuracy is related to the baseline of the system and the resolution of the sensor. As a result, practical considerations tend to limit the application of such systems to remote objects no more than a few metres away. In addition, as the baseline is lengthened, the angle between the incident light pattern and the line of sight of the camera becomes more obtuse and shadows cast by features in the remote object can obscure the 3D profile measurement.
Alternative techniques rely on scanning a device which measures the distance to a remote point; e.g. a laser rangefinder (LRF) or interferometer, using a remote controlled pan and tilt unit.
This scanned LRF approach does not suffer from the range limitations of the structured light approach, but is relatively expensive to implement because high precision and costly components are required for good accuracy. In addition, because the whole mass of the distance measuring device is scanned, achieving scanning at rates sufficient to give “real time” 3D image data is problematic.
The applicant has proposed new techniques for the measurement of range and for the gathering of surface profile data.
EP1 252 535 discloses an optical distance measurement system which uses the cross correlation of a maximal length sequence. EP 1 323 830 discloses the use of similar techniques in a system for obtaining surface profile information.
The surface profile measurement system of EP 1 323 830 uses a pixelated sensor combined with cross correlation distance measurement techniques.
These approaches work well for relatively short ranges, but require much higher levels of illumination power to work at ranges of many kilometres. However, there are applications which do require the determination of 3D surface profiles at such ranges: e.g. military target recognition and remote monitoring of dangerous sites.
A further factor affecting long range surface profile measurement is the effect of atmospheric turbulence, as this causes the apparent position of the far object being examined to move with time.
One approach which is being explored for 3D surface profile measurement is an extension of laser range-gated cameras. As illustrated in FIG. 1, these cameras use a pulsed illuminator and high speed shuttered camera. Timing and control electronics (1) cause the pulsed illuminator (2) to issue a high intensity, very short pulse of illumination at time T0. This travels to the target, a portion of it is reflected by the target and gathered by the lens of the high speed shuttered camera (3). The timing and control electronics then commands the shutter of the high speed camera (3) to open for a short period of time dT at time T1. The image data captured by the camera is presented to the user on a video display (4).
With this arrangement, and assuming that the duration of the illumination pulse is greater than dT, then it can be seen that the high speed shuttered camera only collects an image from the reflected light from the illuminator which has been delayed by travelling over a range of distances R1 to R1+dR where,R1=c·(T1−T0)/2AnddR=c·dT/2where:                c=Speed of light in air (˜3.108 m/s)        T0=start time of illumination pulse        T1=start time of exposure period        dT=duration of exposure period        
Because image data is only collected from light reflected over the distance range R1 to R1 +dR, the camera can be made insensitive to light scattered by the intervening atmosphere, which would otherwise degrade the resolution of the captured image. For example, if (T1−T0)=50 uS and dT=1 uS, then the camera is only sensitive to reflected light from objects between 7500 m and 7600 m.
To achieve these very short exposures, gated intensified cameras are often used with high intensity pulsed lasers providing the illumination.
For 3D imaging, it has been realised that by reducing the duration of the exposure period and collecting a sequence of images where the exposure time is stepped sequentially over a range of values, a set of image “slices”, each corresponding to different depths can be captured and processed to give a 3D surface profile of the remote object.
However, it will be recognised that very short exposure times are required; for example to get a depth resolution of 0.5 m an exposure time and time delay stability of around 3.3 nS are required which requires very expensive and specialised hardware to achieve. In addition, because the exposure time is so short, a very high intensity illumination source (albeit for a short period) is required, which is also expensive.
The significant technical and commercial challenges involved make implementing this approach very difficult.